Selection of coded motion information for LUT updating

ABSTRACT

A method of video decoding includes maintaining a number of tables, wherein each table includes a set of motion candidates, wherein each motion candidate is associated with corresponding motion information derived from previously video blocks, performing a conversion between a video block and a coded representation of the video block, and determining, based on a conversion condition of the video block, whether to update at least one of the tables by adding motion information corresponding to the video block.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of International Application No.PCT/IB2019/055571 filed on Jul. 1, 2019 which claims the priority to andbenefits of International Patent Application No. PCT/CN2018/093663,filed on Jun. 29, 2018. All the aforementioned patent applications arehereby incorporated by reference in their entireties.

TECHNICAL FIELD

This patent document relates to video coding techniques, devices andsystems.

BACKGROUND

In spite of the advances in video compression, digital video stillaccounts for the largest bandwidth use on the internet and other digitalcommunication networks. As the number of connected user devices capableof receiving and displaying video increases, it is expected that thebandwidth demand for digital video usage will continue to grow.

SUMMARY

This document discloses methods, systems, and devices for encoding anddecoding digital video using a merge list of motion vectors.

In one example aspect, a video decoding method is disclosed. The methodincludes maintaining a number of tables, wherein each table includes aset of motion candidates, wherein each motion candidate is associatedwith corresponding motion information derived from previously videoblocks, performing a conversion between a video block and a codedrepresentation of the video block, and determining, based on aconversion condition of the video block, whether to update at least oneof the tables by adding motion information corresponding to the videoblock.

In another example aspect, another video decoding method is disclosed.The method includes maintaining a number of tables, wherein each tableincludes a set of motion candidates, wherein each motion candidate isassociated with corresponding motion information derived from previouslyvideo blocks, performing a conversion between a video block and a codedrepresentation of the video block, selecting M representative positionswithin the video block, where M is an integer, to load motioninformation associated with the M representative positions, and updatingat least one of the tables using motion information associated with theM representative positions.

In yet another example aspect, a video encoder device that implements avideo encoding method described herein is disclosed.

In yet another representative aspect, the various techniques describedherein may be embodied as a computer program product stored on anon-transitory computer readable media. The computer program productincludes program code for carrying out the methods described herein.

In yet another representative aspect, a video decoder apparatus mayimplement a method as described herein.

The details of one or more implementations are set forth in theaccompanying attachments, the drawings, and the description below. Otherfeatures will be apparent from the description and drawings, and fromthe claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing an example of a video encoderimplementation

FIG. 2 illustrates macroblock partitioning in the H.264 video codingstandard.

FIG. 3 illustrates an example of splitting coding blocks (CB) intoprediction blocks (PU).

FIG. 4 illustrates an example implementation for subdivision of a CTBinto CBs and transform block (TBs). Solid lines indicate CB boundariesand dotted lines indicate TB boundaries, including an example CTB withits partitioning, and a corresponding quadtree.

FIG. 5 shows an example of a Quad Tree Binary Tree (QTBT) structure forpartitioning video data.

FIG. 6 shows an example of video block partitioning.

FIG. 7 shows an example of quad-tree partitioning.

FIG. 8 shows an example of tree-type signaling.

FIG. 9 shows an example of a derivation process for merge candidate listconstruction.

FIG. 10 shows example positions of spatial merge candidates.

FIG. 11 shows examples of candidate pairs considered for redundancycheck of spatial merge candidates.

FIG. 12 shows examples of positions for the second PU of N×2N and 2N×Npartitions.

FIG. 13 illustrates motion vector scaling for temporal merge candidates.

FIG. 14 shows candidate positions for temporal merge candidates, andtheir co-located picture.

FIG. 15 shows an example of a combined bi-predictive merge candidate.

FIG. 16 shows an example of a derivation process for motion vectorprediction candidates.

FIG. 17 shows an example of motion vector scaling for spatial motionvector candidates.

FIG. 18 shows an example Alternative Temporal Motion Vector Prediction(ATMVP for motion prediction of a CU.

FIG. 19 pictorially depicts an example of identification of a sourceblock and a source picture.

FIG. 20 shows an example of one CU with four sub-blocks and neighboringblocks.

FIG. 21 illustrates an example of bilateral matching.

FIG. 22 illustrates an example of template matching.

FIG. 23 depicts an example of unilateral Motion Estimation (ME) in FrameRate UpConversion (FRUC).

FIG. 24 shows an example of DMVR based on bilateral template matching.

FIG. 25 shows an example of spatially neighboring blocks used to derivespatial merge candidates.

FIG. 26 depicts an example how selection of a representative positionfor look-up table updates.

FIG. 27 illustrates examples of updating look up table with new set ofmotion information.

FIG. 28 is a block diagram of an example of a hardware platform forimplementing a visual media decoding or a visual media encodingtechnique described in the present document.

FIG. 29 is a flowchart for another example method of video bitstreamprocessing.

FIG. 30 is a flowchart for another example method of video bitstreamprocessing.

DETAILED DESCRIPTION

To improve compression ratio of video, researchers are continuallylooking for new techniques by which to encode video.

1. Introduction

The present document is related to video coding technologies.Specifically, it is related to motion information coding (such as mergemode, AMVP mode) in video coding. It may be applied to the existingvideo coding standard like HEVC, or the standard (Versatile VideoCoding) to be finalized. It may be also applicable to future videocoding standards or video codec.

Brief Discussion

Video coding standards have evolved primarily through the development ofthe well-known ITU-T and ISO/IEC standards. The ITU-T produced H.261 andH.263, ISO/IEC produced MPEG-1 and MPEG-4 Visual, and the twoorganizations jointly produced the H.262/MPEG-2 Video and H.264/MPEG-4Advanced Video Coding (AVC) and H.265/HEVC standards. Since H.262, thevideo coding standards are based on the hybrid video coding structurewherein temporal prediction plus transform coding are utilized. Anexample of a typical HEVC encoder framework is depicted in FIG. 1 .

2.1 Partition Structure

2.1.1 Partition Tree Structure in H.264/AVC

The core of the coding layer in previous standards was the macroblock,containing a 16×16 block of luma samples and, in the usual case of 4:2:0color sampling, two corresponding 8×8 blocks of chroma samples.

An intra-coded block uses spatial prediction to exploit spatialcorrelation among pixels. Two partitions are defined: 16×16 and 4×4.

An inter-coded block uses temporal prediction, instead of spatialprediction, by estimating motion among pictures. Motion can be estimatedindependently for either 16×16 macroblock or any of its sub-macroblockpartitions: 16×8, 8×16, 8×8, 8×4, 4×8, 4×4 (see FIG. 2 ). Only onemotion vector (MV) per sub-macroblock partition is allowed.

2.1.2 Partition Tree Structure in HEVC

In HEVC, a CTU is split into CUs by using a quadtree structure denotedas coding tree to adapt to various local characteristics. The decisionwhether to code a picture area using inter-picture (temporal) orintra-picture (spatial) prediction is made at the CU level. Each CU canbe further split into one, two or four PUs according to the PU splittingtype. Inside one PU, the same prediction process is applied and therelevant information is transmitted to the decoder on a PU basis. Afterobtaining the residual block by applying the prediction process based onthe PU splitting type, a CU can be partitioned into transform units(TUs) according to another quadtree structure similar to the coding treefor the CU. One of key feature of the HEVC structure is that it has themultiple partition conceptions including CU, PU, and TU.

In the following, the various features involved in hybrid video codingusing HEVC are highlighted as follows.

1) Coding tree units and coding tree block (CTB) structure: Theanalogous structure in HEVC is the coding tree unit (CTU), which has asize selected by the encoder and can be larger than a traditionalmacroblock. The CTU consists of a luma CTB and the corresponding chromaCTBs and syntax elements. The size L×L of a luma CTB can be chosen asL=16, 32, or 64 samples, with the larger sizes typically enabling bettercompression. HEVC then supports a partitioning of the CTBs into smallerblocks using a tree structure and quadtree-like signaling.

2) Coding units (CUs) and coding blocks (CBs): The quadtree syntax ofthe CTU specifies the size and positions of its luma and chroma CBs. Theroot of the quadtree is associated with the CTU. Hence, the size of theluma CTB is the largest supported size for a luma CB. The splitting of aCTU into luma and chroma CBs is signaled jointly. One luma CB andordinarily two chroma CBs, together with associated syntax, form acoding unit (CU). A CTB may contain only one CU or may be split to formmultiple CUs, and each CU has an associated partitioning into predictionunits (PUs) and a tree of transform units (TUs).

3) Prediction units and prediction blocks (PBs): The decision whether tocode a picture area using inter picture or intra picture prediction ismade at the CU level. A PU partitioning structure has its root at the CUlevel. Depending on the basic prediction-type decision, the luma andchroma CBs can then be further split in size and predicted from luma andchroma prediction blocks (PBs). HEVC supports variable PB sizes from64×64 down to 4×4 samples. FIG. 3 shows examples of allowed PBs for aM×M CU.

4) TUs and transform blocks: The prediction residual is coded usingblock transforms. A TU tree structure has its root at the CU level. Theluma CB residual may be identical to the luma transform block (TB) ormay be further split into smaller luma TBs. The same applies to thechroma TBs. Integer basis functions similar to those of a discretecosine transform (DCT) are defined for the square TB sizes 4×4, 8×8,16×16, and 32×32. For the 4×4 transform of luma intra picture predictionresiduals, an integer transform derived from a form of discrete sinetransform (DST) is alternatively specified.

FIG. 4 shows an example of a subdivision of a CTB into CBs and transformblock (TBs)]. Solid lines indicate CB borders and dotted lines indicateTB borders. (a) CTB with its partitioning. (b) corresponding quadtree.

2.1.2.1 Tree-Structured Partitioning into Transform Blocks and Units

For residual coding, a CB can be recursively partitioned into transformblocks (TBs). The partitioning is signaled by a residual quadtree. Onlysquare CB and TB partitioning is specified, where a block can berecursively split into quadrants, as illustrated in FIG. 4 . For a givenluma CB of size M×M, a flag signals whether it is split into four blocksof size M/2×M/2. If further splitting is possible, as signaled by amaximum depth of the residual quadtree indicated in the SPS, eachquadrant is assigned a flag that indicates whether it is split into fourquadrants. The leaf node blocks resulting from the residual quadtree arethe transform blocks that are further processed by transform coding. Theencoder indicates the maximum and minimum luma TB sizes that it willuse. Splitting is implicit when the CB size is larger than the maximumTB size. Not splitting is implicit when splitting would result in a lumaTB size smaller than the indicated minimum. The chroma TB size is halfthe luma TB size in each dimension, except when the luma TB size is 4×4,in which case a single 4×4 chroma TB is used for the region covered byfour 4×4 luma TBs. In the case of intra-picture-predicted CUs, thedecoded samples of the nearest-neighboring TBs (within or outside theCB) are used as reference data for intra picture prediction.

In contrast to previous standards, the HEVC design allows a TB to spanacross multiple PBs for inter-picture predicted CUs to maximize thepotential coding efficiency benefits of the quadtree-structured TBpartitioning.

2.1.2.2 Parent and Child Nodes

A CTB is divided according to a quad-tree structure, the nodes of whichare coding units. The plurality of nodes in a quad-tree structureincludes leaf nodes and non-leaf nodes. The leaf nodes have no childnodes in the tree structure (i.e., the leaf nodes are not furthersplit). The, non-leaf nodes include a root node of the tree structure.The root node corresponds to an initial video block of the video data(e.g., a CTB). For each respective non-root node of the plurality ofnodes, the respective non-root node corresponds to a video block that isa sub-block of a video block corresponding to a parent node in the treestructure of the respective non-root node. Each respective non-leaf nodeof the plurality of non-leaf nodes has one or more child nodes in thetree structure.

2.1.3 Quadtree Plus Binary Tree Block Structure with Larger CTUs in JEM

To explore the future video coding technologies beyond HEVC, Joint VideoExploration Team (WET) was founded by VCEG and MPEG jointly in 2015.Since then, many new methods have been adopted by JVET and put into thereference software named Joint Exploration Model (JEM).

2.1.3.1 QTBT Block Partitioning Structure

Different from HEVC, the QTBT structure removes the concepts of multiplepartition types, i.e. it removes the separation of the CU, PU and TUconcepts, and supports more flexibility for CU partition shapes. In theQTBT block structure, a CU can have either a square or rectangularshape. As shown in FIG. 5 , a coding tree unit (CTU) is firstpartitioned by a quadtree structure. The quadtree leaf nodes are furtherpartitioned by a binary tree structure. There are two splitting types,symmetric horizontal splitting and symmetric vertical splitting, in thebinary tree splitting. The binary tree leaf nodes are called codingunits (CUs), and that segmentation is used for prediction and transformprocessing without any further partitioning. This means that the CU, PUand TU have the same block size in the QTBT coding block structure. Inthe JEM, a CU sometimes consists of coding blocks (CBs) of differentcolour components, e.g. one CU contains one luma CB and two chroma CBsin the case of P and B slices of the 4:2:0 chroma format and sometimesconsists of a CB of a single component, e.g., one CU contains only oneluma CB or just two chroma CBs in the case of I slices.

The following parameters are defined for the QTBT partitioning scheme.

-   -   CTU size: the root node size of a quadtree, the same concept as        in HEVC    -   MinQTSize: the minimally allowed quadtree leaf node size    -   MaxBTSize: the maximally allowed binary tree root node size    -   MaxBTDepth: the maximally allowed binary tree depth    -   MinBTSize: the minimally allowed binary tree leaf node size

In one example of the QTBT partitioning structure, the CTU size is setas 128×128 luma samples with two corresponding 64×64 blocks of chromasamples, the MinQTSize is set as 16×16, the MaxBTSize is set as 64×64,the MinBTSize (for both width and height) is set as 4×4, and theMaxBTDepth is set as 4. The quadtree partitioning is applied to the CTUfirst to generate quadtree leaf nodes. The quadtree leaf nodes may havea size from 16×16 (i.e., the MinQTSize) to 128×128 (i.e., the CTU size).If the leaf quadtree node is 128×128, it will not be further split bythe binary tree since the size exceeds the MaxBTSize (i.e., 64×64).Otherwise, the leaf quadtree node could be further partitioned by thebinary tree. Therefore, the quadtree leaf node is also the root node forthe binary tree and it has the binary tree depth as 0. When the binarytree depth reaches MaxBTDepth (i.e., 4), no further splitting isconsidered. When the binary tree node has width equal to MinBTSize(i.e., 4), no further horizontal splitting is considered. Similarly,when the binary tree node has height equal to MinBTSize, no furthervertical splitting is considered. The leaf nodes of the binary tree arefurther processed by prediction and transform processing without anyfurther partitioning. In the JEM, the maximum CTU size is 256×256 lumasamples.

FIG. 5 (left) illustrates an example of block partitioning by usingQTBT, and FIG. 5 (right) illustrates the corresponding treerepresentation. The solid lines indicate quadtree splitting and dottedlines indicate binary tree splitting. In each splitting (i.e., non-leaf)node of the binary tree, one flag is signalled to indicate whichsplitting type (i.e., horizontal or vertical) is used, where 0 indicateshorizontal splitting and 1 indicates vertical splitting. For thequadtree splitting, there is no need to indicate the splitting typesince quadtree splitting always splits a block both horizontally andvertically to produce 4 sub-blocks with an equal size.

In addition, the QTBT scheme supports the ability for the luma andchroma to have a separate QTBT structure. Currently, for P and B slices,the luma and chroma CTBs in one CTU share the same QTBT structure.However, for I slices, the luma CTB is partitioned into CUs by a QTBTstructure, and the chroma CTBs are partitioned into chroma CUs byanother QTBT structure. This means that a CU in an I slice consists of acoding block of the luma component or coding blocks of two chromacomponents, and a CU in a P or B slice consists of coding blocks of allthree colour components.

In HEVC, inter prediction for small blocks is restricted to reduce thememory access of motion compensation, such that bi-prediction is notsupported for 4×8 and 8×4 blocks, and inter prediction is not supportedfor 4×4 blocks. In the QTBT of the JEM, these restrictions are removed.

2.1.4 Ternary-Tree for VVC

In some embodiments, tree types other than quad-tree and binary-tree aresupported. In the implementation, two more ternary tree (TT) partitions,i.e., horizontal and vertical center-side ternary-trees are introduced,as shown in FIGS. 6 (d) and (e).

FIG. 6 shows: (a) quad-tree partitioning (b) vertical binary-treepartitioning (c) horizontal binary-tree partitioning (d) verticalcenter-side ternary-tree partitioning (e) horizontal center-sideternary-tree partitioning.

In some implementations, there are two levels of trees, region tree(quad-tree) and prediction tree (binary-tree or ternary-tree). A CTU isfirstly partitioned by region tree (RT). A RT leaf may be further splitwith prediction tree (PT). A PT leaf may also be further split with PTuntil max PT depth is reached. A PT leaf is the basic coding unit. It isstill called CU for convenience. A CU cannot be further split.Prediction and transform are both applied on CU in the same way as JEM.The whole partition structure is named ‘multiple-type-tree’.

2.1.5 Partitioning Structure

The tree structure used in this response, called Multi-Tree Type (MTT),is a generalization of the QTBT. In QTBT, as shown in FIG. 5 , a CodingTree Unit (CTU) is firstly partitioned by a quad-tree structure. Thequad-tree leaf nodes are further partitioned by a binary-tree structure.

The fundamental structure of MTT constitutes of two types of tree nodes:Region Tree (RT) and Prediction Tree (PT), supporting nine types ofpartitions, as shown in FIG. 7 .

FIG. 7 shows: (a) quad-tree partitioning (b) vertical binary-treepartitioning (c) horizontal binary-tree partitioning (d) verticalternary-tree partitioning (e) horizontal ternary-tree partitioning (f)horizontal-up asymmetric binary-tree partitioning (g) horizontal-downasymmetric binary-tree partitioning (h) vertical-left asymmetricbinary-tree partitioning (i) vertical-right asymmetric binary-treepartitioning.

A region tree can recursively split a CTU into square blocks down to a4×4 size region tree leaf node. At each node in a region tree, aprediction tree can be formed from one of three tree types: Binary Tree(BT), Ternary Tree (TT), and Asymmetric Binary Tree (ABT). In a PTsplit, it is prohibited to have a quadtree partition in branches of theprediction tree. As in JEM, the luma tree and the chroma tree areseparated in I slices. The signaling methods for RT and PT areillustrated in FIG. 8 .

2.2 Inter Prediction in HEVC/H.265

Each inter-predicted PU has motion parameters for one or two referencepicture lists. Motion parameters include a motion vector and a referencepicture index. Usage of one of the two reference picture lists may alsobe signalled using inter_pred_idc. Motion vectors may be explicitlycoded as deltas relative to predictors, such a coding mode is calledAMVP mode.

When a CU is coded with skip mode, one PU is associated with the CU, andthere are no significant residual coefficients, no coded motion vectordelta or reference picture index. A merge mode is specified whereby themotion parameters for the current PU are obtained from neighbouring PUs,including spatial and temporal candidates. The merge mode can be appliedto any inter-predicted PU, not only for skip mode. The alternative tomerge mode is the explicit transmission of motion parameters, wheremotion vector, corresponding reference picture index for each referencepicture list and reference picture list usage are signalled explicitlyper each PU.

When signalling indicates that one of the two reference picture lists isto be used, the PU is produced from one block of samples. This isreferred to as ‘uni-prediction’. Uni-prediction is available both forP-slices and B-slices.

When signalling indicates that both of the reference picture lists areto be used, the PU is produced from two blocks of samples. This isreferred to as ‘bi-prediction’. Bi-prediction is available for B-slicesonly.

The following text provides the details on the inter prediction modesspecified in HEVC. The description will start with the merge mode.

2.2.1 Merge Mode

2.2.1.1 Derivation of Candidates for Merge Mode

When a PU is predicted using merge mode, an index pointing to an entryin the merge candidates list is parsed from the bitstream and used toretrieve the motion information. The construction of this list isspecified in the HEVC standard and can be summarized according to thefollowing sequence of steps:

-   -   Step 1: Initial candidates derivation        -   Step 1.1: Spatial candidates derivation        -   Step 1.2: Redundancy check for spatial candidates        -   Step 1.3: Temporal candidates derivation    -   Step 2: Additional candidates insertion        -   Step 2.1: Creation of bi-predictive candidates        -   Step 2.2: Insertion of zero motion candidates

These steps are also schematically depicted in FIG. 9 . For spatialmerge candidate derivation, a maximum of four merge candidates areselected among candidates that are located in five different positions.For temporal merge candidate derivation, a maximum of one mergecandidate is selected among two candidates. Since constant number ofcandidates for each PU is assumed at decoder, additional candidates aregenerated when the number of candidates does not reach to maximum numberof merge candidate (MaxNumMergeCand) which is signalled in slice header.Since the number of candidates is constant, index of best mergecandidate is encoded using truncated unary binarization (TU). If thesize of CU is equal to 8, all the PUs of the current CU share a singlemerge candidate list, which is identical to the merge candidate list ofthe 2N×2N prediction unit.

In the following, the operations associated with the aforementionedsteps are detailed.

2.2.1.2 Spatial Candidates Derivation

In the derivation of spatial merge candidates, a maximum of four mergecandidates are selected among candidates located in the positionsdepicted in FIG. 10 . The order of derivation is A₁, B₁, B₀, A₀ and B₂.Position B₂ is considered only when any PU of position A₁, B₁, B₀, A₀ isnot available (e.g. because it belongs to another slice or tile) or isintra coded. After candidate at position A₁ is added, the addition ofthe remaining candidates is subject to a redundancy check which ensuresthat candidates with same motion information are excluded from the listso that coding efficiency is improved. To reduce computationalcomplexity, not all possible candidate pairs are considered in thementioned redundancy check. Instead only the pairs linked with an arrowin FIG. 11 are considered and a candidate is only added to the list ifthe corresponding candidate used for redundancy check has not the samemotion information. Another source of duplicate motion information isthe “second PU” associated with partitions different from 2N×2N. As anexample, FIG. 12 depicts the second PU for the case of N×2N and 2N×N,respectively. When the current PU is partitioned as N×2N, candidate atposition A₁ is not considered for list construction. In fact, by addingthis candidate will lead to two prediction units having the same motioninformation, which is redundant to just have one PU in a coding unit.Similarly, position B₁ is not considered when the current PU ispartitioned as 2N×N.

2.2.1.3 Temporal Candidate Derivation

In this step, only one candidate is added to the list. Particularly, inthe derivation of this temporal merge candidate, a scaled motion vectoris derived based on co-located PU belonging to the picture which has thesmallest POC difference with current picture within the given referencepicture list. The reference picture list to be used for derivation ofthe co-located PU is explicitly signalled in the slice header. Thescaled motion vector for temporal merge candidate is obtained asillustrated by the dashed line in FIG. 13 , which is scaled from themotion vector of the co-located PU using the POC distances, tb and td,where tb is defined to be the POC difference between the referencepicture of the current picture and the current picture and td is definedto be the POC difference between the reference picture of the co-locatedpicture and the co-located picture. The reference picture index oftemporal merge candidate is set equal to zero. A practical realizationof the scaling process is described in the HEVC specification. For aB-slice, two motion vectors, one is for reference picture list 0 and theother is for reference picture list 1, are obtained and combined to makethe bi-predictive merge candidate. Illustration of motion vector scalingfor temporal merge candidate.

In the co-located PU (Y) belonging to the reference frame, the positionfor the temporal candidate is selected between candidates C₀ and C₁, asdepicted in FIG. 14 . If PU at position C₀ is not available, is intracoded, or is outside of the current CTU, position C₁ is used. Otherwise,position C₀ is used in the derivation of the temporal merge candidate.

2.2.1.4 Additional Candidates Insertion

Besides spatio-temporal merge candidates, there are two additional typesof merge candidates: combined bi-predictive merge candidate and zeromerge candidate. Combined bi-predictive merge candidates are generatedby utilizing spatio-temporal merge candidates. Combined bi-predictivemerge candidate is used for B-Slice only. The combined bi-predictivecandidates are generated by combining the first reference picture listmotion parameters of an initial candidate with the second referencepicture list motion parameters of another. If these two tuples providedifferent motion hypotheses, they will form a new bi-predictivecandidate. As an example, FIG. 15 depicts the case when two candidatesin the original list (on the left), which have mvL0 and refldxL0 or mvL1and refldxL1, are used to create a combined bi-predictive mergecandidate added to the final list (on the right). There are numerousrules regarding the combinations which are considered to generate theseadditional merge candidates.

Zero motion candidates are inserted to fill the remaining entries in themerge candidates list and therefore hit the MaxNumMergeCand capacity.These candidates have zero spatial displacement and a reference pictureindex which starts from zero and increases every time a new zero motioncandidate is added to the list. The number of reference frames used bythese candidates is one and two for uni and bi-directional prediction,respectively. Finally, no redundancy check is performed on thesecandidates.

2.2.1.5 Motion Estimation Regions for Parallel Processing

To speed up the encoding process, motion estimation can be performed inparallel whereby the motion vectors for all prediction units inside agiven region are derived simultaneously. The derivation of mergecandidates from spatial neighbourhood may interfere with parallelprocessing as one prediction unit cannot derive the motion parametersfrom an adjacent PU until its associated motion estimation is completed.To mitigate the trade-off between coding efficiency and processinglatency, HEVC defines the motion estimation region (MER) whose size issignalled in the picture parameter set using the “log2_parallel_merge_level_minus2” syntax element. When a MER is defined,merge candidates falling in the same region are marked as unavailableand therefore not considered in the list construction.

7.3.2.3 Picture Parameter Set RBSP Syntax

7.3.2.3.1 General Picture Parameter Set RBSP Syntax

Descriptor pic_parameter_set_rbsp( ) { pps_pic_parameter_set_id ue(v)pps_seq_parameter_set_id ue(v) dependent_slice_segments_enabled_flagu(1) ... pps_scaling_list_data_present_flag u(1) if(pps_scaling_list_data_present_flag ) scaling_list_data( )lists_modification_present_flag u(1) log2_parallel_merge_level_minus2ue(v) slice_segment_header_extension_present_flag u(1)pps_extension_present_flag u(1) ... rbsp_trailing_bits( ) }log 2_parallel_merge_level_minus2 plus 2 specifies the value of thevariable Log 2ParMrgLevel, which is used in the derivation process forluma motion vectors for merge mode as specified in clause 8.5.3.2.2 andthe derivation process for spatial merging candidates as specified inclause 8.5.3.2.3. The value of log 2parallel_merge_level_minus2 shall bein the range of 0 to Ctb Log 2SizeY−2, inclusive.

The variable Log 2ParMrgLevel is derived as follows:

Log 2ParMrgLevel=log 2parallel_merge_level_minus2+2 (7-37) NOTE 3—Thevalue of Log 2ParMrgLevel indicates the built-in capability of parallelderivation of the merging candidate lists. For example, when Log2ParMrgLevel is equal to 6, the merging candidate lists for all theprediction units (PUs) and coding units (CUs) contained in a 64×64 blockcan be derived in parallel.

2.2.2 Motion Vector Prediction in AMVP Mode

Motion vector prediction exploits spatio-temporal correlation of motionvector with neighbouring PUs, which is used for explicit transmission ofmotion parameters. It constructs a motion vector candidate list byfirstly checking availability of left, above temporally neighbouring PUpositions, removing redundant candidates and adding zero vector to makethe candidate list to be constant length. Then, the encoder can selectthe best predictor from the candidate list and transmit thecorresponding index indicating the chosen candidate. Similarly withmerge index signalling, the index of the best motion vector candidate isencoded using truncated unary. The maximum value to be encoded in thiscase is 2 (e.g., FIGS. 2 to 8 ). In the following sections, detailsabout derivation process of motion vector prediction candidate areprovided.

2.2.2.1 Derivation of Motion Vector Prediction Candidates

FIG. 16 summarizes derivation process for motion vector predictioncandidate.

In motion vector prediction, two types of motion vector candidates areconsidered: spatial motion vector candidate and temporal motion vectorcandidate. For spatial motion vector candidate derivation, two motionvector candidates are eventually derived based on motion vectors of eachPU located in five different positions as depicted in FIG. 11 .

For temporal motion vector candidate derivation, one motion vectorcandidate is selected from two candidates, which are derived based ontwo different co-located positions. After the first list ofspatio-temporal candidates is made, duplicated motion vector candidatesin the list are removed. If the number of potential candidates is largerthan two, motion vector candidates whose reference picture index withinthe associated reference picture list is larger than 1 are removed fromthe list. If the number of spatio-temporal motion vector candidates issmaller than two, additional zero motion vector candidates is added tothe list.

2.2.2.2 Spatial Motion Vector Candidates

In the derivation of spatial motion vector candidates, a maximum of twocandidates are considered among five potential candidates, which arederived from PUs located in positions as depicted in FIG. 11 , thosepositions being the same as those of motion merge. The order ofderivation for the left side of the current PU is defined as A₀, A₁, andscaled A₀, scaled A₁. The order of derivation for the above side of thecurrent PU is defined as B₀, B₁, B₂, scaled B₀, scaled B₁, scaled B₂.For each side there are therefore four cases that can be used as motionvector candidate, with two cases not required to use spatial scaling,and two cases where spatial scaling is used. The four different casesare summarized as follows.

-   -   No spatial scaling        -   (1) Same reference picture list, and same reference picture            index (same POC)        -   (2) Different reference picture list, but same reference            picture (same POC)    -   Spatial scaling        -   (3) Same reference picture list, but different reference            picture (different POC)        -   (4) Different reference picture list, and different            reference picture (different POC)

The no-spatial-scaling cases are checked first followed by the spatialscaling. Spatial scaling is considered when the POC is different betweenthe reference picture of the neighbouring PU and that of the current PUregardless of reference picture list. If all PUs of left candidates arenot available or are intra coded, scaling for the above motion vector isallowed to help parallel derivation of left and above MV candidates.Otherwise, spatial scaling is not allowed for the above motion vector.

In a spatial scaling process, the motion vector of the neighbouring PUis scaled in a similar manner as for temporal scaling, as depicted asFIG. 17 . The main difference is that the reference picture list andindex of current PU is given as input; the actual scaling process is thesame as that of temporal scaling.

2.2.2.3 Temporal Motion Vector Candidates

Apart for the reference picture index derivation, all processes for thederivation of temporal merge candidates are the same as for thederivation of spatial motion vector candidates (see, e.g., FIG. 6 ). Thereference picture index is signalled to the decoder.

2.2.2.4 Signaling of AMVP Information

For the AMVP mode, four parts may be signalled in the bitstream, i.e.,prediction direction, reference index, MVD and my predictor candidateindex.

Syntax Tables:

Descriptor prediction_unit( x0, y0, nPbW, nPbH ) { if( cu_skip_flag[ x0][ y0 ] ) { if( MaxNumMergeCand > 1 ) merge_idx[ x0 ][ y0 ] ae(v) } else{ /* MODE_INTER */ merge_flag[ x0 ][ y0 ] ae(v) if( merge_flag[ x0 ][ y0] ) { if( MaxNumMergeCand > 1 ) merge_idx[ x0 ][ y0 ] ae(v) } else { if(slice type = = B ) inter_pred_idc[ x0 ][ y0 ] ae(v) if( inter_pred_idc[x0 ][ y0 ] != PRED_L1 ) { if( num_ref_idx_l0_active_minus1 > 0 )ref_idx_l0[ x0 ][ y0 ] ae(v) mvd_coding( x0, y0, 0 ) mvp_l0_flag[ x0 ][y0 ] ae(v) } if( inter_pred_idc[ x0 ][ y0 ] != PRED_L0 ) { if(num_ref_idx_l1_active_minus1 > 0 ) ref_idx_l1[ x0 ][ y0 ] ae(v) if(mvd_l1_zero_flag && inter_pred_idc[ x0 ][ y0 ] = = PRED_BI ) { MvdL1[ x0][ y0 ][ 0 ] = 0 MvdLl [ x0 ][ y0 ][ 1 ] = 0 } else mvd_coding( x0, y0,1 ) mvp_l1_flag[ x0 ][ y0 ] ae(v) } } } }7.3.8.9 Motion Vector Difference Syntax

Descriptor mvd_coding( x0, y0, refList ) { abs_mvd_greater0_flag[ 0 ]ae(v) abs_mvd_greater0 _flag[ 1 ] ae(v) if( abs_mvd_greater0_flag[ 0 ] )abs_mvd_greater1_flag[ 0 ] ae(v) if( abs_mvd_greater0_flag[ 1 ] )abs_mvd_greater1_flag[ 1 ] ae(v) if( abs_mvd_greater0_flag[ 0 ] ) { if(abs_mvd_greater1_flag[ 0 ] ) abs_mvd_minus2[ 0 ] ae(v) mvd_sign_flag[ 0] ae(v) } if( abs_mvd_greater0_flag[ 1 ] ) { if( abs_mvd_greater1_flag[1 ] ) abs_mvd_minus2[ 1 ] ae(v) mvd_sign_flag[ 1 ] ae(v) } }

2.3 New Inter Prediction Methods in JEM (Joint Exploration Model)

2.3.1 Sub-CU Based Motion Vector Prediction

In the JEM with QTBT, each CU can have at most one set of motionparameters for each prediction direction. Two sub-CU level motion vectorprediction methods are considered in the encoder by splitting a large CUinto sub-CUs and deriving motion information for all the sub-CUs of thelarge CU. Alternative temporal motion vector prediction (ATMVP) methodallows each CU to fetch multiple sets of motion information frommultiple blocks smaller than the current CU in the collocated referencepicture. In spatial-temporal motion vector prediction (STMVP) methodmotion vectors of the sub-CUs are derived recursively by using thetemporal motion vector predictor and spatial neighbouring motion vector.

To preserve more accurate motion field for sub-CU motion prediction, themotion compression for the reference frames is currently disabled.

2.3.1.1 Alternative Temporal Motion Vector Prediction

In the alternative temporal motion vector prediction (ATMVP) method, themotion vectors temporal motion vector prediction (TMVP) is modified byfetching multiple sets of motion information (including motion vectorsand reference indices) from blocks smaller than the current CU. As shownin FIG. 18 , the sub-CUs are square N×N blocks (N is set to 4 bydefault).

ATMVP predicts the motion vectors of the sub-CUs within a CU in twosteps. The first step is to identify the corresponding block in areference picture with a so-called temporal vector. The referencepicture is called the motion source picture. The second step is to splitthe current CU into sub-CUs and obtain the motion vectors as well as thereference indices of each sub-CU from the block corresponding to eachsub-CU, as shown in FIG. 18 .

In the first step, a reference picture and the corresponding block isdetermined by the motion information of the spatial neighbouring blocksof the current CU. To avoid the repetitive scanning process ofneighbouring blocks, the first merge candidate in the merge candidatelist of the current CU is used. The first available motion vector aswell as its associated reference index are set to be the temporal vectorand the index to the motion source picture. This way, in ATMVP, thecorresponding block may be more accurately identified, compared withTMVP, wherein the corresponding block (sometimes called collocatedblock) is always in a bottom-right or center position relative to thecurrent CU. In one example, if the first merge candidate is from theleft neighboring block (i.e., A₁ in FIG. 19 ), the associated MV andreference picture are utilized to identify the source block and sourcepicture.

FIG. 19 shows an example of the identification of source block andsource picture

In the second step, a corresponding block of the sub-CU is identified bythe temporal vector in the motion source picture, by adding to thecoordinate of the current CU the temporal vector. For each sub-CU, themotion information of its corresponding block (the smallest motion gridthat covers the center sample) is used to derive the motion informationfor the sub-CU. After the motion information of a corresponding N×Nblock is identified, it is converted to the motion vectors and referenceindices of the current sub-CU, in the same way as TMVP of HEVC, whereinmotion scaling and other procedures apply. For example, the decoderchecks whether the low-delay condition (i.e. the POCs of all referencepictures of the current picture are smaller than the POC of the currentpicture) is fulfilled and possibly uses motion vector MV_(x) (the motionvector corresponding to reference picture list X) to predict motionvector MV_(y) (with X being equal to 0 or 1 and Y being equal to 1−X)for each sub-CU.

2.3.1.2 Spatial-Temporal Motion Vector Prediction

In this method, the motion vectors of the sub-CUs are derivedrecursively, following raster scan order. FIG. 20 illustrates thisconcept. Let us consider an 8×8 CU which contains four 4×4 sub-CUs A, B,C, and D. The neighbouring 4×4 blocks in the current frame are labelledas a, b, c, and d.

The motion derivation for sub-CU A starts by identifying its two spatialneighbours. The first neighbour is the N×N block above sub-CU A (blockc). If this block c is not available or is intra coded the other N×Nblocks above sub-CU A are checked (from left to right, starting at blockc). The second neighbour is a block to the left of the sub-CU A (blockb). If block b is not available or is intra coded other blocks to theleft of sub-CU A are checked (from top to bottom, staring at block b).The motion information obtained from the neighbouring blocks for eachlist is scaled to the first reference frame for a given list. Next,temporal motion vector predictor (TMVP) of sub-block A is derived byfollowing the same procedure of TMVP derivation as specified in HEVC.The motion information of the collocated block at location D is fetchedand scaled accordingly. Finally, after retrieving and scaling the motioninformation, all available motion vectors (up to 3) are averagedseparately for each reference list. The averaged motion vector isassigned as the motion vector of the current sub-CU.

FIG. 20 shows an example of one CU with four sub-blocks (A-D) and itsneighbouring blocks (a-d).

2.3.1.3 Sub-CU Motion Prediction Mode Signalling

The sub-CU modes are enabled as additional merge candidates and there isno additional syntax element required to signal the modes. Twoadditional merge candidates are added to merge candidates list of eachCU to represent the ATMVP mode and STMVP mode. Up to seven mergecandidates are used, if the sequence parameter set indicates that ATMVPand STMVP are enabled. The encoding logic of the additional mergecandidates is the same as for the merge candidates in the HM, whichmeans, for each CU in P or B slice, two more RD checks is needed for thetwo additional merge candidates.

In the JEM, all bins of merge index is context coded by CABAC. While inHEVC, only the first bin is context coded and the remaining bins arecontext by-pass coded.

2.3.2 Adaptive Motion Vector Difference Resolution

In HEVC, motion vector differences (MVDs) (between the motion vector andpredicted motion vector of a PU) are signalled in units of quarter lumasamples when use_integer_mv_flag is equal to 0 in the slice header. Inthe JEM, a locally adaptive motion vector resolution (LAMVR) isintroduced. In the JEM, MVD can be coded in units of quarter lumasamples, integer luma samples or four luma samples. The MVD resolutionis controlled at the coding unit (CU) level, and MVD resolution flagsare conditionally signalled for each CU that has at least one non-zeroMVD components.

For a CU that has at least one non-zero MVD components, a first flag issignalled to indicate whether quarter luma sample MV precision is usedin the CU. When the first flag (equal to 1) indicates that quarter lumasample MV precision is not used, another flag is signalled to indicatewhether integer luma sample MV precision or four luma sample MVprecision is used.

When the first MVD resolution flag of a CU is zero, or not coded for aCU (meaning all MVDs in the CU are zero), the quarter luma sample MVresolution is used for the CU. When a CU uses integer-luma sample MVprecision or four-luma-sample MV precision, the MVPs in the AMVPcandidate list for the CU are rounded to the corresponding precision.

In the encoder, CU-level RD checks are used to determine which MVDresolution is to be used for a CU. That is, the CU-level RD check isperformed three times for each MVD resolution. To accelerate encoderspeed, the following encoding schemes are applied in the JEM.

During RD check of a CU with normal quarter luma sample MVD resolution,the motion information of the current CU (integer luma sample accuracy)is stored. The stored motion information (after rounding) is used as thestarting point for further small range motion vector refinement duringthe RD check for the same CU with integer luma sample and 4 luma sampleMVD resolution so that the time-consuming motion estimation process isnot duplicated three times.

RD check of a CU with 4 luma sample MVD resolution is conditionallyinvoked. For a CU, when RD cost integer luma sample MVD resolution ismuch larger than that of quarter luma sample MVD resolution, the RDcheck of 4 luma sample MVD resolution for the CU is skipped.

2.3.3 Pattern Matched Motion Vector Derivation

Pattern matched motion vector derivation (PMMVD) mode is a special mergemode based on Frame-Rate Up Conversion (FRUC) techniques. With thismode, motion information of a block is not signalled but derived atdecoder side.

A FRUC flag is signalled for a CU when its merge flag is true. When theFRUC flag is false, a merge index is signalled and the regular mergemode is used. When the FRUC flag is true, an additional FRUC mode flagis signalled to indicate which method (bilateral matching or templatematching) is to be used to derive motion information for the block.

At encoder side, the decision on whether using FRUC merge mode for a CUis based on RD cost selection as done for normal merge candidate. Thatis the two matching modes (bilateral matching and template matching) areboth checked for a CU by using RD cost selection. The one leading to theminimal cost is further compared to other CU modes. If a FRUC matchingmode is the most efficient one, FRUC flag is set to true for the CU andthe related matching mode is used.

Motion derivation process in FRUC merge mode has two steps. A CU-levelmotion search is first performed, then followed by a Sub-CU level motionrefinement. At CU level, an initial motion vector is derived for thewhole CU based on bilateral matching or template matching. First, a listof MV candidates is generated and the candidate which leads to theminimum matching cost is selected as the starting point for further CUlevel refinement. Then a local search based on bilateral matching ortemplate matching around the starting point is performed and the MVresults in the minimum matching cost is taken as the MV for the wholeCU. Subsequently, the motion information is further refined at sub-CUlevel with the derived CU motion vectors as the starting points.

For example, the following derivation process is performed for a W×H CUmotion information derivation. At the first stage, MV for the whole W×HCU is derived. At the second stage, the CU is further split into M×Msub-CUs. The value of M is calculated as in (16), D is a predefinedsplitting depth which is set to 3 by default in the JEM. Then the MV foreach sub-CU is derived.

$\begin{matrix}{M = {\max\left\{ {4,\ {\min\left\{ {\frac{M}{2^{D}},\ \frac{N}{2^{D}}} \right\}}} \right\}}} & (1)\end{matrix}$

As shown in the FIG. 21 , the bilateral matching is used to derivemotion information of the current CU by finding the closest matchbetween two blocks along the motion trajectory of the current CU in twodifferent reference pictures. Under the assumption of continuous motiontrajectory, the motion vectors MV0 and MV1 pointing to the two referenceblocks shall be proportional to the temporal distances, i.e., TD0 andTD1, between the current picture and the two reference pictures. As aspecial case, when the current picture is temporally between the tworeference pictures and the temporal distance from the current picture tothe two reference pictures is the same, the bilateral matching becomesmirror based bi-directional MV.

As shown in FIG. 22 , template matching is used to derive motioninformation of the current CU by finding the closest match between atemplate (top and/or left neighbouring blocks of the current CU) in thecurrent picture and a block (same size to the template) in a referencepicture. Except the aforementioned FRUC merge mode, the templatematching is also applied to AMVP mode. In the JEM, as done in HEVC, AMVPhas two candidates. With template matching method, a new candidate isderived. If the newly derived candidate by template matching isdifferent to the first existing AMVP candidate, it is inserted at thevery beginning of the AMVP candidate list and then the list size is setto two (meaning remove the second existing AMVP candidate). When appliedto AMVP mode, only CU level search is applied.

2.3.3.1 CU Level MV Candidate Set

The MV candidate set at CU level consists of:

(i) Original AMVP candidates if the current CU is in AMVP mode

(ii) all merge candidates,

(iii) several MVs in the interpolated MV field.

(iv) top and left neighbouring motion vectors

When using bilateral matching, each valid MV of a merge candidate isused as an input to generate a MV pair with the assumption of bilateralmatching. For example, one valid MV of a merge candidate is (MVa, refa)at reference list A. Then the reference picture refb of its pairedbilateral MV is found in the other reference list B so that refa andrefb are temporally at different sides of the current picture. If such arefb is not available in reference list B, refb is determined as areference which is different from refa and its temporal distance to thecurrent picture is the minimal one in list B. After refb is determined,MVb is derived by scaling MVa based on the temporal distance between thecurrent picture and refa, refb.

Four MVs from the interpolated MV field are also added to the CU levelcandidate list. More specifically, the interpolated MVs at the position(0, 0), (W/2, 0), (0, H/2) and (W/2, H/2) of the current CU are added.

When FRUC is applied in AMVP mode, the original AMVP candidates are alsoadded to CU level MV candidate set.

At the CU level, up to 15 MVs for AMVP CUs and up to 13 MVs for mergeCUs are added to the candidate list.

2.3.3.2 Sub-CU Level MV Candidate Set

The MV candidate set at sub-CU level consists of:

(i) an MV determined from a CU-level search,

(ii) top, left, top-left and top-right neighbouring MVs,

(iii) scaled versions of collocated MVs from reference pictures,

(iv) up to 4 ATMVP candidates,

(v) up to 4 STMVP candidates

The scaled MVs from reference pictures are derived as follows. All thereference pictures in both lists are traversed. The MVs at a collocatedposition of the sub-CU in a reference picture are scaled to thereference of the starting CU-level MV.

ATMVP and STMVP candidates are limited to the four first ones.

At the sub-CU level, up to 17 MVs are added to the candidate list.

2.3.3.3 Generation of Interpolated MV Field

Before coding a frame, interpolated motion field is generated for thewhole picture based on unilateral ME. Then the motion field may be usedlater as CU level or sub-CU level MV candidates.

First, the motion field of each reference pictures in both referencelists is traversed at 4×4 block level. For each 4×4 block, if the motionassociated to the block passing through a 4×4 block in the currentpicture (as shown in FIG. 23 ) and the block has not been assigned anyinterpolated motion, the motion of the reference block is scaled to thecurrent picture according to the temporal distance TD0 and TD1 (the sameway as that of MV scaling of TMVP in HEVC) and the scaled motion isassigned to the block in the current frame. If no scaled MV is assignedto a 4×4 block, the block's motion is marked as unavailable in theinterpolated motion field.

2.3.3.4 Interpolation and Matching Cost

When a motion vector points to a fractional sample position, motioncompensated interpolation is needed. To reduce complexity, bi-linearinterpolation instead of regular 8-tap HEVC interpolation is used forboth bilateral matching and template matching.

The calculation of matching cost is a bit different at different steps.When selecting the candidate from the candidate set at the CU level, thematching cost is the absolute sum difference (SAD) of bilateral matchingor template matching. After the starting MV is determined, the matchingcost C of bilateral matching at sub-CU level search is calculated asfollows:C=SAD+w·(|MV _(x) −MV _(x) ^(s) |+|MV _(y) −MV _(y) ^(s))  (2)

where w is a weighting factor which is empirically set to 4, MV andMV^(s) indicate the current MV and the starting MV, respectively. SAD isstill used as the matching cost of template matching at sub-CU levelsearch.

In FRUC mode, MV is derived by using luma samples only. The derivedmotion will be used for both luma and chroma for MC inter prediction.After MV is decided, final MC is performed using 8-taps interpolationfilter for luma and 4-taps interpolation filter for chroma.

2.3.3.5 MV Refinement

MV refinement is a pattern based MV search with the criterion ofbilateral matching cost or template matching cost. In the JEM, twosearch patterns are supported—an unrestricted center-biased diamondsearch (UCBDS) and an adaptive cross search for MV refinement at the CUlevel and sub-CU level, respectively. For both CU and sub-CU level MVrefinement, the MV is directly searched at quarter luma sample MVaccuracy, and this is followed by one-eighth luma sample MV refinement.The search range of MV refinement for the CU and sub-CU step are setequal to 8 luma samples.

2.3.3.6 Selection of Prediction Direction in Template Matching FRUCMerge Mode

In the bilateral matching merge mode, bi-prediction is always appliedsince the motion information of a CU is derived based on the closestmatch between two blocks along the motion trajectory of the current CUin two different reference pictures. There is no such limitation for thetemplate matching merge mode. In the template matching merge mode, theencoder can choose among uni-prediction from list0, uni-prediction fromlist1 or bi-prediction for a CU. The selection is based on a templatematching cost as follows:

-   -   If costBi<=factor*min (cost0, cost1)        -   bi-prediction is used;    -   Otherwise, if cost0<=cost1        -   uni-prediction from list0 is used;    -   Otherwise,        -   uni-prediction from list1 is used;

where cost0 is the SAD of list0 template matching, cost1 is the SAD oflist1 template matching and costBi is the SAD of bi-prediction templatematching. The value of factor is equal to 1.25, which means that theselection process is biased toward bi-prediction. The inter predictiondirection selection is only applied to the CU-level template matchingprocess.

2.3.4 Decoder-Side Motion Vector Refinement

In bi-prediction operation, for the prediction of one block region, twoprediction blocks, formed using a motion vector (MV) of list0 and a MVof list1, respectively, are combined to form a single prediction signal.In the decoder-side motion vector refinement (DMVR) method, the twomotion vectors of the bi-prediction are further refined by a bilateraltemplate matching process. The bilateral template matching applied inthe decoder to perform a distortion-based search between a bilateraltemplate and the reconstruction samples in the reference pictures inorder to obtain a refined MV without transmission of additional motioninformation.

In DMVR, a bilateral template is generated as the weighted combination(i.e. average) of the two prediction blocks, from the initial MV0 oflist0 and MV1 of list1, respectively, as shown in FIG. 23 . The templatematching operation consists of calculating cost measures between thegenerated template and the sample region (around the initial predictionblock) in the reference picture. For each of the two reference pictures,the MV that yields the minimum template cost is considered as theupdated MV of that list to replace the original one. In the JEM, nine MVcandidates are searched for each list. The nine MV candidates includethe original MV and 8 surrounding MVs with one luma sample offset to theoriginal MV in either the horizontal or vertical direction, or both.Finally, the two new MVs, i.e., MV0′ and MV1′ as shown in FIG. 24 , areused for generating the final bi-prediction results. A sum of absolutedifferences (SAD) is used as the cost measure.

DMVR is applied for the merge mode of bi-prediction with one MV from areference picture in the past and another from a reference picture inthe future, without the transmission of additional syntax elements. Inthe JEM, when LIC, affine motion, FRUC, or sub-CU merge candidate isenabled for a CU, DMVR is not applied.

2.3.5 Merge/Skip Mode with Bilateral Matching Refinement

A merge candidate list is first constructed by inserting the motionvectors and reference indices of the spatial neighboring and temporalneighboring blocks into the candidate list with redundancy checkinguntil the number of the available candidates reaches the maximumcandidate size of 19. The merge candidate list for the merge/skip modeis constructed by inserting spatial candidates (FIG. 11 ), temporalcandidates, affine candidates, advanced temporal MVP (ATMVP) candidate,spatial temporal MVP (STMVP) candidate and the additional candidates asused in HEVC (Combined candidates and Zero candidates) according to apre-defined insertion order:

-   -   Spatial candidates for blocks 1-4.    -   Extrapolated affine candidates for blocks 1-4.    -   ATMVP.    -   STMVP.    -   Virtual affine candidate.    -   Spatial candidate (block 5) (used only when the number of the        available candidates is smaller than 6).    -   Extrapolated affine candidate (block 5).    -   Temporal candidate (derived as in HEVC).    -   Non-adjacent spatial candidates followed by extrapolated affine        candidate (blocks 6 to 49, as depicted in FIG. 25 ).    -   Combined candidates.    -   Zero candidates

It is noted that IC flags are also inherited from merge candidatesexcept for STMVP and affine. Moreover, for the first four spatialcandidates, the bi-prediction ones are inserted before the ones withuni-prediction.

In some implementations, blocks which are not connected with the currentblock may be accessed. If a non-adjacent block is coded with non-intramode, the associated motion information may be added as an additionalmerge candidate.

3. Examples of Problems Addressed by Embodiments Disclosed Herein

The current HEVC design could take the correlation of current block itsneighbouring blocks (next to the current block) to better code themotion information. However, it is possible that that the neighbouringblocks correspond to different objects with different motiontrajectories. In this case, prediction from its neighbouring blocks isnot efficient.

Prediction from motion information of non-adjacent blocks could bringadditional coding gain with the cost of storing all the motioninformation (typically on 4×4 level) into cache which significantlyincrease the complexity for hardware implementation.

4. Some Examples

To overcome the drawbacks of existing implementations, LUT-based motionvector prediction techniques using one or more look up tables with atleast one motion candidate stored to predict motion information of ablock can be implemented in various embodiments to provide video codingwith higher coding efficiencies. Each LUT can include one or more motioncandidates, each associated with corresponding motion information.Motion information of a motion candidate can include partial or all ofthe prediction direction, reference indices/pictures, motion vectors,LIC flags, affine flags, Motion Vector Derivation (MVD) precisions,and/or MVD values. Motion information may further include the blockposition information to indicate wherein the motion information iscoming from.

The LUT-based motion vector prediction based on the disclosedtechnology, which may enhance both existing and future video codingstandards, is elucidated in the following examples described for variousimplementations. Because the LUTs allow the encoding/decoding process tobe performed based on historical data (e.g., the blocks that have beenprocessed), the LUT-based motion vector prediction can also be referredto as History-based Motion Vector Prediction (HMVP) method. In theLUT-based motion vector prediction method, one or multiple tables withmotion information from previously coded blocks are maintained duringthe encoding/decoding process. During the encoding/decoding of oneblock, the associated motion information in LUTs may be added to themotion candidate lists, and after encoding/decoding one block, LUTs maybe updated. The examples below should be considered as examples toexplain general concepts. These examples should not be interpreted in anarrow way. Furthermore, these examples can be combined in any manner.

Some embodiments may use one or more look up tables with at least onemotion candidate stored to predict motion information of a block.Embodiments may use motion candidate to indicate a set of motioninformation stored in a look up table. For conventional AMVP or mergemodes, embodiments may use AMVP or merge candidates for storing themotion information.

The examples below explain general concepts.

Examples of Look-Up Tables

Example A1: Each look up table may contain one or more motion candidateswherein each candidate is associated with its motion information.

-   -   a. Motion information of a motion candidate here may include        partial or all of the prediction direction, reference        indices/pictures, motion vectors, LIC flag, affine flag, MVD        precision, MVD values.    -   b. Motion information may further include the block position        information to indicate wherein the motion information is coming        from.    -   c. A counter may be further assigned for each look up table.        -   i. The counter may be initialized to be zero at the            beginning of encoding/decoding a picture/slice/LCU(CTU)            row/tile.        -   ii. In one example, the counter may be updated after            encoding/decoding a CTU/CTB/CU/CB/PU/a certain region size            (e.g., 8×8 or 16×16).        -   iii. In one example, the counter is increased by one each            time one candidate is added into the lookup table.        -   iv. In one example, the counter should be no larger than the            table size (number of allowed motion candidates).        -   v. Alternatively, the counter may be used to indicate how            many motion candidates have been tried to be added to the            look up tables (some of them was in the look up table but            later may be removed from the table). In this case, the            counter could be larger than the table size.    -   d. The table size (number of allowed motion candidates) and/or        number of tables may be the fixed or adaptive. The table size        may be same for all tables, or different for different tables.        -   i. Alternatively, different sizes may be used for different            look-up tables (e.g., 1 or 2).        -   ii. In one example, the table sizes and/or number of tables            may be pre-defined.        -   iii. In one example, the table sizes and/or number of tables            may be signalled in Video Parameter Set (VPS), Sequence            Parameter Set (SPS), Picture Parameter Set (PPS), Slice            header, tile header, Coding Tree Unit (CTU), Coding Tree            Block (CTB), Coding Unit (CU) or Prediction Unit (PU),            region covering multiple CTU/CTB/CU/PUs.        -   iv. The table size and/or number of tables may further            depend on the slice type, temporal layer index of a picture,            picture order count (POC) distance between one slice and the            closest intra slice.    -   e. Suppose there are N tables used for a coding thread, N*P        tables may be required for coding a slice, wherein P indicates        the number of LCU rows or the number of tiles.        -   i. Alternatively, only P tables may be required for coding a            slice, wherein P indicates the number of LCU rows wherein            each LCU row only use one look up table even N could be            larger than 1 when tile is disabled.

Selection of LUTs

Example B1

For coding a block, partial or all of motion candidates from one look uptable may be checked in order. When one motion candidate is checkedduring coding a block, it may be added to the motion candidate list(e.g., AMVP, merge candidate lists).

-   -   a. Alternatively, motion candidates from multiple look up tables        may be checked in order.    -   b. The look up table indices may be signaled in CTU, CTB, CU or        PU, or a region covering multiple CTU/CTB/CU/PUs.

Example B2

The selection of look up tables may depend on the position of a block.

-   -   a. It may depend on the CTU address covering the block. Here, we        take two look up tables (Dual Look Up Tables, DLUT) for an        example to illustrate the idea:        -   i. If the block is located at one of the first M CTUs in a            CTU row, the first look up table may be utilized for coding            the block, while for blocks located in the remaining CTUs in            the CTU row, the second look up table may be utilized.        -   ii. If the block is located at one of the first M CTUs in a            CTU row, motion candidates of the first look up table may            firstly checked for coding the block, if there are not            enough candidates in the first table, the second look up            table may be further utilized. while for blocks located in            the remaining CTUs in the CTU row, the second look up table            may be utilized.        -   iii. Alternatively, for blocks located in the remaining CTUs            in the CTU row, motion candidates of the second look up            table may firstly checked for coding the block, if there are            not enough candidates in the second table, the first look up            table may be further utilized.    -   b. It may depend on the distance between the position of the        block and the position associated with one motion candidate in        one or multiple look up tables.        -   iv. In one example, if one motion candidate is associated            with a smaller distance to the block to be coded, it may be            checked earlier compared to another motion candidate.

Usage of Look Up Tables

Example C1

The total number of motion candidates in a look up table to be checkedmay be pre-defined.

-   -   a. It may further depend on the coded information, block size,        block shape and etc. al. For example, for the AMVP mode, only m        motion candidates may be checked while for the merge mode, n        motion candidates may be checked (e.g., m=2, n=44).    -   b. In one example, the total number of motion candidates to be        checked may be signalled in Video Parameter Set (VPS), Sequence        Parameter Set (SPS), Picture Parameter Set (PPS), Slice header,        tile header, Coding Tree Unit (CTU), Coding Tree Block (CTB),        Coding Unit (CU) or Prediction Unit (PU), region covering        multiple CTU/CTB/CU/PUs.

Example C2

The motion candidate(s) included in a look up table may be directlyinherited by a block.

-   -   a. They may be used for the merge mode coding, i.e., motion        candidates may be checked in the merge candidate list derivation        process.    -   b. They may be used for the affine merge mode coding.        -   i. A motion candidate in a look up table can be added as an            affine merge candidate if its affine flag is one.    -   c. Checking of motion candidates in look up tables may be        enabled when:        -   i. the merge candidate list is not full after inserting the            TMVP candidate;        -   ii. the merge candidate list is not full after checking a            certain spatial neighboring block for spatial merge            candidate derivation;        -   iii. the merge candidate list is not full after all spatial            merge candidates;        -   iv. the merge candidate list is not full after combined            bi-predictive merge candidates;        -   v. Pruning may be applied before adding a motion candidate            to the merge candidate list.

Example C3

The motion candidate(s) included in a look up table may be used as apredictor for coding motion information of a block.

-   -   a. They may be used for the AMVP mode coding, i.e., motion        candidates may be checked in the AMVP candidate list derivation        process.    -   b. Checking of motion candidates in look up tables may be        enabled when:        -   i. the AMVP candidate list is not full after inserting the            TMVP candidate;        -   ii. the AMVP candidate list is not full after selecting from            spatial neighbors and pruning, right before inserting the            TMVP candidate;        -   iii. when there is no AMVP candidate from above neighboring            blocks without scaling and/or when there is no AMVP            candidate from left neighboring blocks without scaling        -   iv. Pruning may be applied before adding a motion candidate            to the AMVP candidate list.    -   c. Motion candidates with identical reference picture to the        current reference picture is checked.        -   i. Alternatively, in addition, motion candidates with            different reference pictures from the current reference            picture are also checked (with MV scaled).        -   ii. Alternatively, all motion candidates with identical            reference picture to the current reference picture are first            checked, then, motion candidates with different reference            pictures from the current reference picture are checked.        -   iii. Alternatively, motion candidates are checked following            the same in merge.

Example C4

The checking order of motion candidates in a look up table is defined asfollows (suppose K (K>=1) motion candidates are allowed to be checked):

-   -   a. The last K motion candidates in the look up table, e.g., in        descending order of entry indices to the LUT.    -   b. The first K % L candidates wherein L is the look up table        size when K>=L, e.g., in descending order of entry indices to        the LUT.    -   c. All the candidates (L candidates) in the look up table when        K>=L.    -   d. Alternatively, furthermore, based on the descending order of        motion candidate indices.    -   e. Alternatively, selecting K motion candidates based on the        candidate information, such as the distance of positions        associated with the motion candidates and current block.    -   f. The checking order of different look up tables is defined in        usage of look up tables in the next subsection.    -   g. The checking process will terminate once the merge/AMVP        candidate list reaches the maximumly allowed candidate numbers.    -   h. Alternatively, it will terminate once the number of added        motion candidates reaches the maximumly allowed motion candidate        numbers.    -   i. One syntax element to indicate the table size as well as the        number of motion candidates (i.e., K=L) allowed to be checked        may be signaled in SPS, PPS, Slice header, tile header.

Example C5

Enabling/disabling the usage look up tables for motion informationcoding of a block may be signalled in SPS, PPS, Slice header, tileheader, CTU, CTB, CU or PU, region covering multiple CTU/CTB/CU/PUs.

Example C6

Whether to apply prediction from look up tables may further depend onthe coded information. When it is inferred not to apply for a block,additional signaling of indications of the prediction is skipped.Alternatively, when it is inferred not to apply for a block, there is noneed to access motion candidates of look up tables, and the checking ofrelated motion candidates is omitted.

-   -   a. Whether to apply prediction from look up tables may depend on        block size/block shape. In one example, for smaller blocks, such        as 4×4, 8×4 or 4×8 blocks, it is disallowed to perform        prediction from look up tables.    -   b. Whether to apply prediction from look up tables may depend on        whether the block is coded with AMVP or merge mode. In one        example, for the AMVP mode, it is disallowed to perform        prediction from look up tables.    -   c. Whether to apply prediction from look up tables may depend on        the block is coded with affine motion or other kinds of motion        (such as translational motion). In one example, for the affine        mode, it is disallowed to perform prediction from look up        tables.

Example C7

Motion candidates of a look up table in previously codedframes/slices/tiles may be used to predict motion information of a blockin a different frame/slice/tile.

-   -   a. In one example, only look up tables associated with reference        pictures of current block may be utilized for coding current        block.    -   b. In one example, only look up tables associated with pictures        with the same slice type and/or same quantization parameters of        current block may be utilized for coding current block.

Update of Look Up Tables

Example D1

After coding a block with motion information (i.e., IntraBC mode, intercoded mode), one or multiple look up tables may be updated.

-   -   a. In one example, whether to update a look up table may reuse        the rules for selecting look up tables, e.g., when a look up        table could be selected for coding the current block, after        coding/decoding the block, the selected look up table may        further be updated.    -   b. Look up tables to be updated may be selected based on coded        information, and/or positions of the block/LCU.    -   c. If the block is coded with motion information directly        signaled (such as AMVP mode), the motion information for the        block may be added to a look up table.        -   i. Alternatively, if the block is coded with motion            information directly inherited from a spatial neighboring            block without any refinement (e.g., spatial merge candidate            without refinement), the motion information for the block            shouldn't be added to a look up table.        -   ii. Alternatively, if the block is coded with motion            information directly inherited from a spatial neighboring            block with refinement (such as DMVR, FRUC), the motion            information for the block shouldn't be added to any look up            table.        -   iii. Alternatively, if the block is coded with motion            information directly inherited from a motion candidate            stored in a look up table, the motion information for the            block shouldn't be added to any look up table.    -   d. M (M>=1) representative position within the block is chosen        and the motion information associated with the representative is        used to update look up tables.        -   i. In one example, the representative position is defined as            one of the four corner positions (e.g., C0-C3 in FIG. 26 )            within the block.        -   ii. In one example, the representative position is defined            as the center position (e.g., Ca−Cd in FIG. 26 ) within the            block.        -   iii. When sub-block prediction is disallowed for block, M is            set to 1.        -   iv. When sub-block prediction is allowed for block, M could            be set to 1 or total number of sub-blocks or any other value            between number of sub-blocks] exclusively.        -   v. Alternatively, when sub-block prediction is allowed for            block, M could be set to 1 and the selection of a            representative sub-block is based on            -   1. the frequency of utilized motion information,        -   2. whether it is a bi-prediction block        -   3. based on the reference picture index/reference picture        -   4. motion vector differences compared to other motion            vectors (e.g., selecting the maximum MV differences)        -   5. other coded information.    -   e. When M (M>=1) sets of representative positions are selected        to update look up tables, further conditions may be checked        before adding them as additional motion candidates to look up        tables.        -   i. Pruning may be applied to the new sets of motion            information to the existing motion candidates in the look up            table.        -   ii. In one example, a new set of motion information            shouldn't be identical to any or partial of existing motion            candidates in the look up table.        -   iii. Alternatively, for same reference pictures from a new            set of motion information and one existing motion candidate,            the MV difference should be no smaller than one/multiple            thresholds. For example, horizontal and/or vertical            component of the MV difference should be larger than 1-pixel            distance.        -   iv. Alternatively, the new sets of motion information are            only pruned with the last K candidates or the first K % L            existing motion candidates when K>L to allow reactivating            the old motion candidates.        -   v. Alternatively, no pruning is applied.    -   f. If M sets of motion information are used to update a look up        table, the corresponding counter should be increased by M.    -   g. Suppose a counter of a look up table to be updated is denoted        by K before coding the current block, after coding the block,        for one selected set of motion information (with methods        mentioned above 0, it is added as an additional motion candidate        with index equal to K % L (wherein L is the look up table size).        Examples are shown in FIG. 27 .        -   i. Alternatively, it is added as an additional motion            candidate with index equal to min(K+1, L−1). Alternatively,            furthermore, if K>=L, the first motion candidate (index            equal to 0) is removed from the look-up table, and the            following K candidates indices are reduced by 1.    -   h. The look-up table may be emptied after coding one        intra-constrained block.    -   i. If an entry of motion information is added into the lookup        table, more entries of motion information may also be added into        the table by derivation from the motion information. In this        case, the counter associated with the look up table may be        increased more than 1.        -   i. In one example, the MV of an entry of motion information            is scaled and put into the table;        -   ii. In one example, the MV of an entry of motion information            is added by (dx, dy) and put into the table;        -   iii. In one example, the average of MVs of two or more            entries of motion information is calculated and put into the            table.

Example D2

If one block is located at a picture/slice/tile border, updating of lookup tables may be always disallowed.

Example D3

Motion information of above LCU rows may be disabled to code the currentLCU row.

-   -   a. In this case, at the beginning of a new slice/tile/LCU row,        the number of available motion candidates may be reset to 0.

Example D4

At the beginning of coding a slice/tile with a new temporal layer index,the number of available motion candidates may be reset to 0.

Example D5

The look up table may be continuously updated with one slice/tile/LCUrow/slices with same temporal layer index.

-   -   a. Alternatively, the look up table may be updated only after        coding/decoding each S (S>=1) CTUs/CTBs/CUs/CBs or after        coding/decoding a certain region (e.g., size equal to 8×8 or        16×16).    -   b. Alternatively, one look up table may stop updating once it        reaches a maximumly allowed counter.    -   c. In one example, the counter may be predefined. Alternatively,        it be signalled in Video Parameter Set (VPS), Sequence Parameter        Set (SPS), Picture Parameter Set (PPS), Slice header, tile        header, Coding Tree Unit (CTU), Coding Tree Block (CTB), Coding        Unit (CU) or Prediction Unit (PU), region covering multiple        CTU/CTB/CU/PUs.

FIG. 28 is a block diagram of a video processing apparatus 2800. Theapparatus 2800 may be used to implement one or more of the methodsdescribed herein. The apparatus 2800 may be embodied in a smartphone,tablet, computer, Internet of Things (IoT) receiver, and so on. Theapparatus 2800 may include one or more processors 2802, one or morememories 2804 and video processing hardware 2806. The processor(s) 2802may be configured to implement one or more methods described in thepresent document. The memory (memories) 2804 may be used for storingdata and code used for implementing the methods and techniques describedherein. The video processing hardware 2806 may be used to implement, inhardware circuitry, some techniques described in the present document.

FIG. 29 is a flowchart for an example of a video decoding method 2900.The method 2900 includes maintaining (2902) a number of tables (e.g.,look-up tables; LUTs), wherein each table includes a set of motioncandidates, wherein each motion candidate is associated withcorresponding motion information derived from previously video blocks,performing (2904) a conversion between a video block and a codedrepresentation of the video block, and determining (2906), based on aconversion condition of the video block, whether to update at least oneof the tables by adding motion information corresponding to the videoblock.

FIG. 30 is a flowchart for an example of video decoding method 3000. Themethod 3000 includes maintaining (3002) a number of tables (e.g.,look-up tables; LUTs), wherein each table includes a set of motioncandidates, wherein each motion candidate is associated withcorresponding motion information derived from previously video blocks,performing (3004) a conversion between a video block and a codedrepresentation of the video block, selecting (3006) M representativepositions within the video block, where M is an integer, to load motioninformation associated with the M representative positions, and updating(3008) at least one of the tables using motion information associatedwith the M representative positions.

From the foregoing, it will be appreciated that specific embodiments ofthe presently disclosed technology have been described herein forpurposes of illustration, but that various modifications may be madewithout deviating from the scope of the invention. Accordingly, thepresently disclosed technology is not limited except as by the appendedclaims.

The disclosed and other embodiments, modules and the functionaloperations described in this document can be implemented in digitalelectronic circuitry, or in computer software, firmware, or hardware,including the structures disclosed in this document and their structuralequivalents, or in combinations of one or more of them. The disclosedand other embodiments can be implemented as one or more computer programproducts, i.e., one or more modules of computer program instructionsencoded on a computer readable medium for execution by, or to controlthe operation of, data processing apparatus. The computer readablemedium can be a machine-readable storage device, a machine-readablestorage substrate, a memory device, a composition of matter effecting amachine-readable propagated signal, or a combination of one or morethem. The term “data processing apparatus” encompasses all apparatus,devices, and machines for processing data, including by way of example aprogrammable processor, a computer, or multiple processors or computers.The apparatus can include, in addition to hardware, code that creates anexecution environment for the computer program in question, e.g., codethat constitutes processor firmware, a protocol stack, a databasemanagement system, an operating system, or a combination of one or moreof them. A propagated signal is an artificially generated signal, e.g.,a machine-generated electrical, optical, or electromagnetic signal, thatis generated to encode information for transmission to suitable receiverapparatus.

A computer program (also known as a program, software, softwareapplication, script, or code) can be written in any form of programminglanguage, including compiled or interpreted languages, and it can bedeployed in any form, including as a stand-alone program or as a module,component, subroutine, or other unit suitable for use in a computingenvironment. A computer program does not necessarily correspond to afile in a file system. A program can be stored in a portion of a filethat holds other programs or data (e.g., one or more scripts stored in amarkup language document), in a single file dedicated to the program inquestion, or in multiple coordinated files (e.g., files that store oneor more modules, sub programs, or portions of code). A computer programcan be deployed to be executed on one computer or on multiple computersthat are located at one site or distributed across multiple sites andinterconnected by a communication network.

The processes and logic flows described in this document can beperformed by one or more programmable processors executing one or morecomputer programs to perform functions by operating on input data andgenerating output. The processes and logic flows can also be performedby, and apparatus can also be implemented as, special purpose logiccircuitry, e.g., an FPGA (field programmable gate array) or an ASIC(application specific integrated circuit).

Processors suitable for the execution of a computer program include, byway of example, both general and special purpose microprocessors, andany one or more processors of any kind of digital computer. Generally, aprocessor will receive instructions and data from a read only memory ora random-access memory or both. The essential elements of a computer area processor for performing instructions and one or more memory devicesfor storing instructions and data. Generally, a computer will alsoinclude, or be operatively coupled to receive data from or transfer datato, or both, one or more mass storage devices for storing data, e.g.,magnetic, magneto optical disks, or optical disks. However, a computerneed not have such devices. Computer readable media suitable for storingcomputer program instructions and data include all forms of non-volatilememory, media and memory devices, including by way of examplesemiconductor memory devices, e.g., EPROM, EEPROM, and flash memorydevices; magnetic disks, e.g., internal hard disks or removable disks;magneto optical disks; and CD ROM and DVD-ROM disks. The processor andthe memory can be supplemented by, or incorporated in, special purposelogic circuitry.

While this patent document contains many specifics, these should not beconstrued as limitations on the scope of any invention or of what may beclaimed, but rather as descriptions of features that may be specific toparticular embodiments of particular inventions. Certain features thatare described in this patent document in the context of separateembodiments can also be implemented in combination in a singleembodiment. Conversely, various features that are described in thecontext of a single embodiment can also be implemented in multipleembodiments separately or in any suitable subcombination. Moreover,although features may be described above as acting in certaincombinations and even initially claimed as such, one or more featuresfrom a claimed combination can in some cases be excised from thecombination, and the claimed combination may be directed to asubcombination or variation of a subcombination.

Similarly, while operations are depicted in the drawings in a particularorder, this should not be understood as requiring that such operationsbe performed in the particular order shown or in sequential order, orthat all illustrated operations be performed, to achieve desirableresults. Moreover, the separation of various system components in theembodiments described in this patent document should not be understoodas requiring such separation in all embodiments.

Only a few implementations and examples are described and otherimplementations, enhancements and variations can be made based on whatis described and illustrated in this patent document.

What is claimed is:
 1. A method for processing video data, comprising:maintaining one or multiple tables, wherein each table includes one ormultiple motion candidates derived from one or more video blocks thathave been coded, and arrangement of the motion candidates in the eachtable is based on a sequence of addition of the motion candidates intothe each table, each motion candidate comprises motion informationassociated with M representative positions of the video block that hasbeen coded; performing a conversion between a current video block and acoded representation of the current video block at least based on amotion candidate list; determining M representative positions within thecurrent video block, where M is an integer, to load motion informationassociated with the M representative positions; and updating at leastone of the one or multiple tables using motion information associatedwith the M representative positions of the current video block; whereinthe updated table used to construct a motion candidate list for asubsequent block, and wherein the motion candidate list is used forderiving motion information of the subsequent block; wherein, in a casesub-block predictions are allowed for processing of the current videoblock, a value of M is set to 1, and the M representative positions areselected based on at least one of a frequency of utilized motioninformation, whether a bi-prediction block is used, a reference picture,a reference picture index, or motion vector differences compared toother motion vectors.
 2. The method of claim 1, wherein the motioncandidate list for the current video block is constructed at least basedon the one or multiple tables.
 3. The method of claim 1, wherein themotion information associated with the M representative positionscomprises M sets of motion information that respectively corresponds tothe M representative positions.
 4. The method of claim 1, wherein the Mrepresentative positions include at least one of four corner positionswithin the current video block, or the M representative positionsinclude at least one of four center positions within the current videoblock.
 5. The method of claim 1, wherein a value of M is set to 1 in acase sub-block predictions are disallowed for the processing of thecurrent video block.
 6. The method of claim 1, wherein M is pre-definedper a Coding Tree Unit or a Coding Tree Block Unit or a Coding Unit or aPrediction Unit and is included in a Video Parameter Set, a SequenceParameter Set, a Picture Parameter Set, a Slice Header, or a tileheader.
 7. The method of claim 1, wherein the motion informationincludes at least one of: a prediction direction, a reference pictureindex, motion vector values, intensity compensation flag, affine flag,motion vector difference precision, and motion vector difference value.8. The method of claim 1, further comprising, reconstructing a secondvideo block different from the current video block based on the updatedone or multiple of tables.
 9. The method of claim 1, wherein a motioncandidate is associated with intra prediction modes used for intra-codedblocks, or the motion candidate is associated with multiple illuminationcompensation (IC) parameters used for IC-coded blocks, or the motioncandidate is associated with filter parameters used in a filteringprocess.
 10. An apparatus for processing video data comprising aprocessor and a non-transitory memory with instructions thereon, whereinthe instructions upon execution by the processor, cause the processorto: maintain one or multiple tables, wherein each table includes one ormultiple motion candidates derived from one or more video blocks thathave been coded, and arrangement of the motion candidates in the eachtable is based on a sequence of addition of the motion candidates intothe each table, each motion candidate comprises motion informationassociated with M representative positions of the video block that hasbeen coded; perform a conversion between a current video block and acoded representation of the current video block at least based on amotion candidate list; determine M representative positions within thecurrent video block, where M is an integer, to load motion informationassociated with the M representative positions; and update at least oneof the one or multiple tables using motion information associated withthe M representative positions of the current video block; wherein theupdated table used to construct a motion candidate list for a subsequentblock, and wherein the motion candidate list is used for deriving motioninformation of the subsequent block; wherein, in a case sub-blockpredictions are allowed for processing of the current video block, avalue of M is set to 1, and the M representative positions are selectedbased on at least one of a frequency of utilized motion information,whether a bi-prediction block is used, a reference picture, a referencepicture index, or motion vector differences compared to other motionvectors.
 11. The apparatus of claim 10, wherein the motion candidatelist for the current video block is constructed at least based on theone or multiple tables.
 12. The apparatus of claim 10, wherein themotion information associated with the M representative positionscomprises M sets of motion information that respectively corresponds tothe M representative positions.
 13. The apparatus of claim 10, whereinthe M representative positions include at least one of four cornerpositions within the current video block, or the M representativepositions include at least one of four center positions within thecurrent video block.
 14. The apparatus of claim 10, wherein a value of Mis set to 1 in a case sub-block predictions are disallowed for theprocessing of the current video block.
 15. The apparatus of claim 10,wherein M is pre-defined per a Coding Tree Unit or a Coding Tree BlockUnit or a Coding Unit or a Prediction Unit and is included in a VideoParameter Set, a Sequence Parameter Set, a Picture Parameter Set, aSlice Header, or a tile header.
 16. A non-transitory computer-readablestorage medium storing instructions that cause a processor to: maintainone or multiple tables, wherein each table includes one or multiplemotion candidates derived from one or more video blocks that have beencoded, and arrangement of the motion candidates in the each table isbased on a sequence of addition of the motion candidates into the eachtable, each motion candidate comprises motion information associatedwith M representative positions of the video block that has been coded;perform a conversion between a current video block and a codedrepresentation of the current video block at least based on a motioncandidate list; determine M representative positions within the currentvideo block, where M is an integer, to load motion informationassociated with the M representative positions; and update at least oneof the one or multiple tables using motion information associated withthe M representative positions of the current video block; wherein theupdated table used to construct a motion candidate list for a subsequentblock, and wherein the motion candidate list is used for deriving motioninformation of the subsequent block; wherein, in a case sub-blockpredictions are allowed for processing of the current video block, avalue of M is set to 1, and the M representative positions are selectedbased on at least one of a frequency of utilized motion information,whether a bi-prediction block is used, a reference picture, a referencepicture index, or motion vector differences compared to other motionvectors.
 17. The apparatus of claim 10, wherein the motion informationincludes at least one of: a prediction direction, a reference pictureindex, motion vector values, intensity compensation flag, affine flag,motion vector difference precision, and motion vector difference value.18. The apparatus of claim 10, wherein the instructions upon executionby the processor, further cause the processor to: reconstruct a secondvideo block different from the current video block based on the updatedone or multiple of tables.
 19. The apparatus of claim 10, wherein amotion candidate is associated with intra prediction modes used forintra-coded blocks, or the motion candidate is associated with multipleillumination compensation (IC) parameters used for IC-coded blocks, orthe motion candidate is associated with filter parameters used in afiltering process.
 20. The non-transitory computer-readable storagemedium of claim 16, wherein the motion candidate list for the currentvideo block is constructed at least based on the one or multiple tables.